Interpretive Summary: Fungi are a group of organisms that cause billions of dollars damage each year to agricultural and forest resources in the United States. One group of fungi includes the species that caused chestnut blight in the eastern United States killing all of the chestnut trees. Many additional fungi belong to the same group and cause diseases of forest trees. This research describes and illustrates all fungal species in a genus related to the chestnut blight fungus. Many of these species occur on hardwood trees in North America and some cause serious diseases such as walnut anthracnose and butternut canker. This research provides descriptions and illustrations of these fungi along with a key for identification to the 45 species in this genus. Many of the species are new to science and do not occur in the United States. Relationships between these species have been determined based on DNA sequences. This paper will be used by forest pathologists to control the diseases caused by these fungi through the accurate identification of the causal organism. In addition, plant quarantine personnel will use this research to formulate regulations about how to keep these fungal pathogens out of the United States.

Technical Abstract:
Species of Ophiognomonia are leaf-inhabiting endophytes, pathogens, and saprobes that infect plants in the families Betulaceae, Fagaceae, Juglandaceae, Lauraceae, Malvaceae, Platanaceae, Rosaceae, Salicaceae, and Sapindaceae. Based on extensive collecting, this species-rich genus is now known to have a worldwide distribution in primarily temperate areas, although some species are known from the subtropics. Analyses of DNA sequences from three markers including guanine nucleotide-binding protein subunit beta-like protein (MS204), translation elongation factor 1a (tef-1a), and the ITS region including ITS1, 5.8S rDNA and ITS2 regions (ITS) were used to define phylogenetic species in Ophiognomonia. Host plant association correlated with these species. Twenty-five new species of Ophiognomonia and two new combinations are proposed with descriptions and illustrations. In addition, descriptions and illustrations are provided for 12 other species of Ophiognomonia. A key is provided to the 45 currently accepted species of Ophiognomonia. The disposition of additional names in Ophiognomonia is also discussed.